1. Technical Field
The present invention relates to a zoom lens used as a projection lens for a projector apparatus and the like. In particular, the invention relates to a wide-angle projection zoom lens suitable for projecting an original image, which is formed by a light valve such as a liquid crystal display element or a DMD (digital micro mirror device), onto a screen in an enlarged manner. And, the present invention relates to a projection type display apparatus equipped with the zoom lens.
2. Related Art
Recently, so-called front projection type projector apparatuses for projecting an image onto a screen in front of the apparatus are widely used for school education, corporate job training, presentation, and the like.
In the projection lenses provided in the front projection type projector apparatuses, there is a demand that the lenses have a compact configuration, a wide angle of view, and a zoom function in consideration of adaptability to an installation condition and mobility of the apparatus.
In order to cope with the demand, there are proposed projection zoom lenses disclosed in the following Patent Document 1 (JP-A-2003-015038), Patent Document 2 (JP-A-2005-292260) and Patent Document 3 (JP-A-2008-309897).
In the lenses disclosed in Patent Documents 1 and 2, a half angle of view thereof at the wide-angle end is less than 45 degrees. However, recently, there has been a demand for a wide-angle projection zoom lens having a half angle of view of 45 degrees or more at the wide-angle end while maintaining favorable optical performance.
In fixed-magnification projection lenses provided in rear projection apparatuses, a lens having a half angle of view of 45 degrees or more has been known. However, in the case of a zoom lens, when the half angle of view of 45 degrees or more is intended to be achieved, it is difficult to maintain a favorable optical performance from the wide-angle end to the telephoto end. In particular, it is also difficult to correct satisfactorily on-axis aberrations such as spherical aberration common to angles of view (image heights) and off-axis aberrations such as distortion and image field curvature varied with the angles of view while balancing these aberrations. Further, there is also a problem in that the size of the lens system of the lens group disposed close to the magnification side increases and thus it is difficult to achieve compactness. For this reason, the demanded zoom lens has not been embodied.
On the other hand, the lens disclosed in Patent Document 3 is a projection type zoom lens having a speed of F1.8, a half angle of view ω of 45 degrees or more at the wide-angle end, and a zoom of 1.3 times. However, in such a zoom lens, if a higher speed is intended to be achieved, it becomes extremely difficult to design the optical system thereof. In particular, a projector set is required to secure a relative illumination of around 80%, and there are many cases where only the projection lens is required to secure a relative illumination of 50%. If the ratio of the relative illumination is intended to be increased, as an angle of view thereof becomes wider, it becomes more difficult to improve resolution performance in the periphery thereof. Thus, this causes an increase in costs.
In addition, in order to improve optical performance, for example, two aspheric lenses may be disposed in the lens system. However, unlike a spherical lens, sometimes an aspheric lens may be required to have eccentricity adjusted after assembly thereof since it is difficult to perform centering on each single element of the lens. Hence, there has been a demand for simplification in optical adjustment.